1. Field of the Invention
The invention concerns a method for generating an activating algorithm processing the sensor signal of an angular rate sensor provided within the safety system of a motor vehicle, by means of which an activation decision for activating at least one safety device of the safety system is taken in relation to the sensor signal, and with the sensor signal representing a measure for the rotational velocity of the rolling motion occurring when a rollover is imminent. Furthermore this invention also concerns a safety system for motor vehicles with at least one safety device, which safety system utilizes this activating algorithm. Here, in connection with rollover events, the safety devices in question are mainly roll bars, seat-belt tensioners, and side airbags.
2. Description of the Related Technology
From EP 0 430 813 B1 a safety system for motor vehicles is known which features an electronic arrangement for controlling at least one safety device in the event of a motor vehicle rolling over. The safety system comprises a gyrometer (angular rate or gyro sensor), which measures the rotational velocity of the rolling motion, and acceleration meters, with the electronic arrangement processing the signals coming from the gyrometer and the acceleration sensors in order to control the activation of the safety device. Here, the sensor signals are evaluated by integration for a specified period of time. In order to avoid an overflow of the integration, the ratio of the transverse acceleration to the vertical acceleration is additionally calculated, and if a specified threshold value is exceeded, integration is released.
The disadvantage of the activating algorithm used in this safety system is above all that, in order to avoid an overflow of the integration, further signals are required in addition to the signals of the angular rate sensor, namely acceleration sensor signals; these only meet the purpose to be able to evaluate the angular rate sensor signals, but entail high manufacturing costs for the total system.
The object of the present invention is to provide a method for generating an activating algorithm processing the sensor signal of an angular rate sensor provided within the safety system of a motor vehicle, by means of which an activation decision for activating at least one safety device of the safety system is taken in relation to the sensor signal, and with the sensor signal representing a measure for the rotational velocity of the rolling motion occurring when a rollover is imminent, and which does not feature the above-mentioned disadvantages, that is, it does not require signals from further expensive acceleration sensors for evaluating the angular rate sensor signals.
According to the present invention, this object is achieved according to the following steps:
a) generating a theoretical characteristic rollover curve as follows:
xcex1th(xcfx89)=xe2x88x92(xcex1tip/xcfx89lim)xcfx89+xcex1tip, xcfx89xe2x89xa70xe2x80x83xe2x80x83Equation (1) 
where xcfx89 corresponds to the initial rotational velocity of a rolling motion of the vehicle, and xcex1th(xcfx89) represents the inclination angle of the vehicle, the constants xcex1tip and xcfx89lim are determined in relation to the actual vehicle and state the static tip angle of the vehicle, which xe2x80x94if exceededxe2x80x94causes the vehicle to tip over, and the rotational velocity range, at which with xcfx89xe2x89xa7xcfx89lim a rollover of the vehicle will occur, and the range Bth which with (xcfx89, xcex1)xe2x80x94combinations with |xcex1|xe2x89xa7xcex1th(|xcfx89|) wherein xcex1 and xcfx89 are real values, represents the associated rollover risk range where a positive activation decision is expected, and
b) generating an activation algorithm by approximating the characteristic rollover curve of Equation (1) in the first quadrant of a coordinate system with at least two low pass filter functions (Y1,n, n=1,2, . . . ; Y2,n, n=1,2, . . . ) each having at least one activation threshold (S1, S2), by the limit frequencies fg1, fg2) of the two low pass filter functions (Y1,n, n=1,2, . . . ; Y2,n, n=1,2, . . . ) and determining said activation thresholds (S1, S2) such that for the range BF2 of the (|xcfx89|, Yl,n, (xcfx89)) value pairs and the range BF2 of the (|xcfx89|, Y2,n, (xcfx89)) value pairs with
|Yl,n(xcfx89)| greater than S1, and |Y2,n(xcfx89)| greater than S2xe2x80x83xe2x80x83Equation (2) 
and wherein Y1,n(xcfx89) and Y2,n(xcfx89) are real values, and BF1⊂Bth and BF2⊂Bth are satisfied.
Due to the fact that with the method according to the invention an activating algorithm is generated, which evaluates with low pass filter functions the sensor signals generated by the angular rate sensor, the disadvantages occurring when integrators are used have been avoided in a surprisingly simple fashion as the measures required to avoid an overflowxe2x80x94in particular, the additional sensors specified in the state of the artxe2x80x94are now no longer required.
The method according to the invention uses a theoretical characteristic rollover curve as a characteristic curve model for generating the activating algorithm in accordance with the invention. The activation algorithm satisfies equation (1) set forth above. The xcfx89-xcex1-graph 1 of the equation (1) is shown in FIG. 1, wherein |xcfx89| represents the rotational velocity value of the rolling motion occurring in the x direction or longitudinal axis of the vehicle if a vehicle rollover is imminent and wherein |xcex1| represents the inclination angle value in the y direction or cross-axis of the vehicle. The xcfx89-xcex1 graph subdivides the first quadrant of a coordinate system into two areas which on the one hand concern vehicle conditions with xcfx89-xcex1-combinations that are to lead to the activation of a safety device, i.e. xe2x80x9cfirexe2x80x9d scenarios, and, on the other hand, xe2x80x9cno-firexe2x80x9d scenarios whose xcfx89-xcex1-combinations are not to lead to the activation of the safety device. The xcfx89lim, 0-combination or 0-xcex1tipxe2x80x94combination represents a limit condition of a vehicle with a rotational velocity xcfx89lim in the x direction and an inclination angle of 0xc2x0 or with a rotational velocity 0 and an inclination angle (static tip angle) xcex1tip that leads to a rollover. These parameters are vehicle specific and, therefore, need to be determined for each vehicle type.
In addition to the theoretical characteristic curve 1, FIG. 1 shows three rollover scenarios with the curves 2, 3, and 4. Curve 2 shows the course of a rollover starting with a high initial velocity. Curve 3 shows a situation where the vehicle is driven onto a screwramp with a following rollover. Curve 4 shows a quasi-static rollover where the vehicle reaches the static tip angle with an angular velocity of almost zero and then rolls over.
The low pass filter functions can be generated as digital filters of the 1st magnitude by means of a computer where the filter algorithm consists of linear difference equations with constant coefficients, and where recursive as well as non-recursive difference equations can be used.
A filter algorithm for the first and second low pass filter function Y1,n, n=1,2, . . . and Y2,n, n=1,2, . . . of a recursive filter of the first magnitude here takes on the following form:
Y1,n=d1Y1,nxe2x88x921+c1Xnxe2x88x921, n=1,2 . . . or 
Y2,n=d2Y2,nxe2x88x921+c2Xnxe2x88x921, n=1,2 . . . 
with Xn representing the digitized input sequence of the rotational velocity xcfx89, and Y1,n or Y2,n representing the corresponding output sequence, that is the inclination angle xcex1 in binary representation. The coefficients of the filter algorithm are determined in accordance with method step (b) by approximation and defining the limit frequencies and the trigger thresholds such that condition (2) is met.
The cutoff frequency (=limit frequency) is defined respectively according to the initial rotational velocity, so that an initially high rotational velocity corresponding to a high initial rotation energy leads to a high cutoff frequency with an appropriately adapted trigger or activation threshold, that is, to a trigger threshold which is also high. A low cutoff frequency is defined for a low initial rotational velocity, which corresponds to a low initial rotation energy, with an appropriately adapted trigger or activation threshold, that is, a trigger threshold which is also low. The values of the cutoff frequency, as well as the relevant trigger threshold, depend on the respective vehicle type as well as on the fitted safety device in the vehicle (roll bar, seat-belt tensioner, side airbag) to be triggered in the event of a crash, and therefore need to be specifically adapted to each application case in order to ensure an optimum and safe trigger behavior. The terms xe2x80x9ctriggerxe2x80x9d and xe2x80x9cactivationxe2x80x9d or xe2x80x9ctrigger thresholdxe2x80x9d and xe2x80x9cactivation thresholdxe2x80x9d are used herein synonymously.
For the approximation of a characteristic rollover curve in accordance with equation (1), a step function xcfx89(t) will preferably be processed as the input sequence for the low pass filter functions; and the resulting xcfx89-xcex1-graph is compared with the graph of the rollover characteristic, and, if necessary, an adaptation of limit frequencies and threshold values is effected.
In addition, the activating algorithm is tested by means of sensor signatures obtained from earlier vehicle tests, i.e. real sensor signatures, and/or such sensor signatures simulated within the framework of a suitable simulation environment. Using these simulation results, the trigger characteristic in the xcfx89-xcex1-graph is evaluated, and, if necessary, an adaptation of the limit frequencies and the associated trigger thresholds is effected.
Using such a method in accordance with the invention to generate an activating algorithm, the trigger behavior can be adapted individually to each vehicle type without having first to carry out costly drive tests.
In a further advantageous embodiment of the method according to the invention, additional signals of further sensors are processed by the activating algorithm, hereinafter designated as extended activating algorithm, with these sensors detecting the vehicle-condition-specific parameters indicating stability, in particular vertical acceleration, lateral acceleration, and inclination angle. Using these additional data, the trigger threshold values can be adapted dynamically to the respective vehicle condition. Thus, for example, the initial value of the inclination angle of the vehicle, or its stability due to the acceleration value in z direction can be taken into account for the activation decision. This is to achieve, in relation to the detected vehicle specific parameters, an even better differentiation according to fire scenariosxe2x80x94i.e. vehicle conditions that lead to a safety device being activatedxe2x80x94and no-fire-scenarios.
The activating algorithm generated by the method according to the invention can be used to advantage within a safety system for motor vehicles. Here, this activating algorithm is implemented in the control unit for the safety system, which features an angular rate sensor for detecting the rotational velocity of the rolling motion of the vehicle, and at least one safety device. The activating algorithm generated according to the invention can be implemented in analog fashion, that is, with the corresponding analog filters, or by means of software using a processor in the control unit of the safety system. less than  less than 
The limit frequencies and the trigger thresholds are defined such that the low pass filter function for the high rotation velocities features the higher cutoff frequency and a correspondingly higher trigger threshold whilst the low pass filter function for the lower rotation velocities also requires a correspondingly lower cutoff frequency as well as a lower trigger threshold. Relevant cutoff frequency fg2 values for a slow rollover are below 5 Hz for fg2. Relevant cutoff frequency fg1 values for a fast rollover are below 10 Hz for fg1.
Thus, in accordance with a particularly preferred example embodiment, a first low pass filter function features a high cutoff frequency fg1 less than 7 Hz and a correspondingly high trigger threshold value in order to detect a fast rollover with a high initial rotation energy, whilst the. The second low pass filter function features a cutoff frequency of fg2 less than 3 Hz with a correspondingly adapted trigger threshold value in order to detect a slow rollover with a lower rotation energy. In the event of fast rollovers in particular, this provides for a fast activation of the safety device. Preferably, this embodiment can be provided with a third low pass filter function, by means of which slow rollovers are detected and which thus features a cutoff frequency fg3 less than 0.5 Hz with an appropriately adapted trigger threshold value. If three low pass filter functions are used, it is possible to achieve an optimum differentiation according to rollover scenarios, and, also according to so-called no-fire scenarios, i.e. vehicle conditions that do not lead to a safety device being activated.
Furthermore, in an advantageous embodiment the signals generated by the angular rate sensor can first be fed into a high pass filter before being processed by the low pass filter functions. In advantageous fashion, this measure reduces the zero point imprecision of the angular rate sensor so that, in particular if several low pass filter functions are used, even rollover events with very low angular rates can be detected.
In a further preferred embodiment of the safety system, the extended activating algorithm is implemented in its control system in order to evaluate vehicle-specific parameters in addition to the angular rate. Preferably, using an acceleration sensor, the vertical acceleration of a motor vehicle is detected and compared with at least one adaptation threshold; and, if this adaptation threshold is exceeded, or if the actual value falls below this threshold, the trigger threshold values will be increased or decreased. Thus, the signal of such an acceleration sensor is not required for the evaluation of the angular rate sensor signalxe2x80x94as provided for by the state of the artxe2x80x94but for the dynamic adaptation of the trigger thresholds as this signal provides additional information regarding the stability of the vehicle and thus carries out some kind of a plausibility check, for example with regard to a high initial rotational velocity value. That is, that for a high value of the z acceleration signal the trigger thresholds can be set higher in spite of a high initial rotational velocity, whilst a low signal indicates a low driving stability of the vehicle and thus requires that a low trigger threshold is set. In advantageous fashion, this achieves a faster activation in the event of slow rollovers, and, at the same time, in the event of extreme situations hardly ever occurring in normal driving conditions, such as e.g. in extremely steep turns, prevents such activation.
Instead of such an acceleration sensor, it is also possible to use an inclination sensor. The adaptation of the trigger thresholds is effected such that, for a large inclination angle, a low trigger threshold is set, as the driving stability will then be low; however, for a lower inclination angle, a higher trigger threshold is to be provided. In addition, the inclination sensor provides the advantage that the sign of the inclination angle can be stated. This allows the thresholds to be adapted asymmetrically, that is, if the angular rate and the inclination angle have the same sign, then a lower trigger threshold value is set, whilst if the signs are different a high trigger threshold value is set.
As further vehicle specific parameters, in another preferred embodiment of the safety system according to the invention, the vertical acceleration as well as the lateral acceleration can be detected, with a dynamic adaptation of the trigger threshold values being effected by means of the quotient from lateral acceleration and vertical acceleration, on the one hand, by setting a low trigger threshold if a high value of this quotient indicates an instable vehicle condition, and, on the other hand, direct and immediate activation of the safety device if the value of this quotient exceeds a predefined fixed quotient threshold. With this type of embodiment, an improved differentiation according to fire scenarios and no-fire scenarios can be achieved.
In a further preferred embodiment, instead of an acceleration transducer for detecting the lateral acceleration, an inclination sensor can be used to measure the inclination angle, with the dynamic trigger threshold values being set by means of the vehicle condition characterizing the vertical acceleration and the rotational velocity. In particular, the sensor values of the inclination sensor can be used to compare the measured values with a tip angle corresponding to the static tip angle of the motor vehicle, in order to trigger directly the safety device if the static tip angle is exceeded. This ensures that if such a rollover scenario occurs, i.e. in the event of a static rollover, the safety device is always activated.
Furthermore, in another advantageous embodiment, the plausibility of the inclination angle can be evaluated by means of the vehicle condition characterizing the vertical acceleration and the rotational velocity, so that, for a plausible inclination angle value, this is set as the actual current value of the inclination angle, with this value that is evaluated as being plausible being compared at the same time with a tip threshold value corresponding to the static tip angle of the motor vehicle and the safety device being activated if the tip threshold value exceeds the amount of this value. The plausibility check is advantageous therefore as it allows driving situations that represent no-fire scenarios, such as when driving through a steep turn, to be easily detected, whilst at the same time for extremely slow rolloversxe2x80x94so-called quasi-static rolloversxe2x80x94where trigger threshold values are not exceededxe2x80x94activation is effected if the tip threshold value is exceeded.
However, if there is no plausible inclination angle value, the change in the inclination angle which occurs during motor vehicle operation is determined by means of an integration of the rotational velocity, and then added to the start angle; the sum is then set as the current inclination angle.
Finally, in dependence of the preset current inclination angle value, the trigger threshold values can be adapted to the vehicle condition characterized by this inclination angle.